Explain the process of breakdown of glucose in a cell: $(i)$ In the presence of oxygen. (ii) In the absence of oxygen.

(N/A) Step $1$: The first step in all pathways is the breakdown of glucose, a $6$-carbon molecule, into a $3$-carbon molecule called pyruvate. This process occurs in the cytoplasm.
Step $2$: In the presence of oxygen (aerobic respiration), pyruvate is broken down in the mitochondria into $CO_2$, $H_2O$, and a large amount of energy.
Step $3$: In the absence of oxygen (anaerobic respiration, e.g., in yeast), pyruvate is converted into ethanol $(C_2H_5OH)$, $CO_2$, and energy.
$\text{Glucose} \rightarrow \text{Pyruvate}$
$\text{Pyruvate} \xrightarrow[\text{of oxygen}]{\text{In presence}} \text{CO}_2 + \text{H}_2\text{O} + \text{Energy}$
$\text{Pyruvate} \xrightarrow[\text{of oxygen}]{\text{In absence}} \text{C}_2\text{H}_5\text{OH} + \text{CO}_2 + \text{Energy}$